Electrode for electrolysis device, electrolysis device, and method for generating electrolysis product material

ABSTRACT

The present disclosure provides an electrode for an electrolysis device, which allows performance of a catalyst to be efficiently exhibited in an electrochemical reaction of reducing an electrolysis reactant to generate an electrolysis product material. Specifically, a carbon fiber that has a structure in which the carbon fiber contains a part of and/or a whole of a catalyst particle is used as a cathode electrode to greatly improve adherence force of the catalyst particle and enable efficient generation of an electrolysis product material.

BACKGROUND

1. Technical Field

The present disclosure relates to a cathode electrode including a regionof a carbon fiber having a structure in which the carbon fiber containsa part of and/or a whole of a catalyst particle and to a method forgenerating an electrolysis product material with the electrode.

2. Description of the Related Art

Development of a technique of forming a catalyst into fine particles andmaking a carrier such as a carbon carry the fine particles on thecarrier has been conducted for the purpose of increasing a specific areaof a catalyst used for an electrochemical reaction and enhancingreaction efficiency per weight of a catalyst.

Patent Literature 1 discloses a method for making a surface of a carbontube membrane carry fine particles containing a carbon dioxidedecomposing element by an electrochemical method and using the elementfor electrochemical reduction of carbon dioxide.

Patent Literature 2 discloses a method for making a surface of a carbonnanofiber carry a catalyst for a battery by a colloidal method.

Patent Literature 3 discloses a method for making a carbon materialcarry a transition metal on a surface of the carbon material by a heattreatment.

CITATION LIST Patent Literatures

PTL 1: Unexamined Japanese Patent Publication No. 2010-255018

PTL 2: WO 2009/003848

PTL 3: Unexamined Japanese Patent Publication No. 2002-83604

PTL 4: Unexamined Japanese Patent Publication No. 2012-209193

PTL 5: WO 2009/140381

PTL 6: Unexamined Japanese Patent Publication No. 2010-118269

PTL 7: U.S. Patent Publication No. 2011/0143253

The methods disclosed in Patent Literatures 1, 2 and 3 have a problemthat the catalyst is carried only on the surface of the carbon material,and therefore adherence of the catalyst with the carbon material is low,resulting in insufficient exhibition of performance of the catalystparticles.

Particularly, when current efficiency is estimated from a result ofelectrochemical reduction of carbon dioxide disclosed in PatentLiterature 1, the current efficiency of a hydrocarbon as an electrolysisproduct material derived from the catalyst is 8.5%, which is lower thanthe current efficiency (40% to 50%) when the catalyst itself is used.

SUMMARY

One non-limiting and exemplary embodiment provides a cathode electrodecapable of efficiently generating an electrolysis product material byusing a carbon fiber having a structure in which the carbon fibercontains a part of and/or a whole of a catalyst particle (i.e.,catalyst-containing structure), to enhance adherence of the catalystparticle with the carbon fiber.

In one general aspect, the techniques disclosed here feature a methodfor reducing a reactant electrochemically with an electrolysis device togenerate a product material, the method comprising:

(a) preparing the electrolysis device comprising a cathode chamber, ananode chamber, a cathode electrode, an anode electrode, and a solidelectrolyte membrane;

wherein

the cathode electrode includes a carbon fiber;

the carbon fiber includes a first catalyst particle and a secondcatalyst particle;

an inside of the carbon fiber is filled with carbon;

at least a part of the first catalyst particle is located on a surfaceof the carbon fiber;

the second catalyst particle is located in the inside of the carbonfiber so as to be surrounded by the carbon with which the inside of thecarbon fiber is filled;

the anode electrode has a region formed of a metal or metal compound;

a first electrolyte solution is stored in the cathode chamber;

a second electrolyte solution is stored in the anode chamber;

the cathode electrode is in contact with the first electrolyte solution;

the anode electrode is in contact with the second electrolyte solution;

the first electrolyte solution contains the reactant; and

the solid electrolyte membrane separates the anode chamber from thecathode chamber; and

(b) applying a voltage between the anode electrode and the cathodeelectrode to reduce the reactant on at least one of the first catalystparticle and the second catalyst particle.

An electrolysis device comprising a cathode electrode according to thepresent disclosure which includes a carbon fiber having acatalyst-containing structure can efficiently generate an electrolysisproduct material.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

It should be noted that general or specific embodiments may beimplemented as a method, an electrolysis device, a cathode electrode orany selective combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an electrolysis device according to a firstexemplary embodiment of the present disclosure;

FIG. 2 is a view showing an electrolysis device according to a secondexemplary embodiment of the present disclosure;

FIG. 3 is a diagram showing pretreatment conditions for a carbon fiberin Examples;

FIG. 4A is a scanning electron micrograph of a surface of a carbonfiber;

FIG. 4B is a transmission electron micrograph of a surface of a carbonfiber;

FIG. 5 is a micrograph showing a surface of an electrolyzed carbon fiberin Example 1;

FIG. 6A is a scanning electron micrograph before an experiment inComparative Example 2;

FIG. 6B is a scanning electron micrograph after an experiment inComparative Example 2;

FIG. 7 is a graph showing a generation amount of a product material perunit time in Examples and Comparative Examples;

FIG. 8A is a graph showing a generation amount of carbon monoxide perunit time when gold particles are used;

FIG. 8B is a graph showing a generation amount of formic acid per unittime when gallium oxide particles are used;

FIG. 8C is a graph showing a generation amount of hydrogen per unit timewhen platinum particles are used;

FIG. 9A is a schematic view of carbon fiber 20; and

FIG. 9B is a sectional view taken along the line 9B-9B.

DETAILED DESCRIPTION

Hereinafter, the present disclosure is described specifically accordingto the exemplary embodiments with reference to the drawings.

First Exemplary Embodiment

FIG. 1 is electrolysis device 100 according to a first exemplaryembodiment of the present disclosure, which is an electrolysis devicethat generates an electrolysis product material from an electrolysisreactant, and includes cathode chamber 12 for storing first electrolytesolution 11 containing an electrolysis reactant; electrode 13 for anelectrolysis device, as a cathode electrode that is disposed in thecathode chamber so as to be in contact with the first electrolytesolution and includes a region of a carbon fiber having a structure inwhich the carbon fiber contains a part of and/or a whole of a catalystparticle; solid electrolyte membrane 16 that separates the cathodechamber from anode chamber 15 for storing second electrolyte solution14; anode electrode 17 that is disposed in the anode chamber so as to bein contact with the second electrolyte solution and has a region formedof a metal or metal compound; and external power source 18 for applyinga voltage so that the cathode electrode has a negative potential withrespect to a potential of the anode electrode.

According to the first exemplary embodiment, an electrolysis productmaterial can be obtained.

Second Exemplary Embodiment

FIG. 2 is electrolysis device 200 according to a second exemplaryembodiment of the present disclosure, which is an electrolysis devicethat generates an electrolysis product material from an electrolysisreactant, and includes cathode chamber 12 for storing first electrolytesolution 11 containing an electrolysis reactant; electrode 13 for anelectrolysis device, as a cathode electrode that is disposed in thecathode chamber so as to be in contact with the first electrolytesolution and that includes a region of a carbon fiber having a structurein which the carbon fiber contains a part of and/or a whole of acatalyst particle; solid electrolyte membrane 16 that separates thecathode chamber from anode chamber 15 for storing second electrolytesolution 14; anode electrode 17 that is disposed in the anode chamber soas to be in contact with the second electrolyte solution and has aregion formed of a metal or metal compound; external power source 18 forapplying a voltage so that the cathode electrode has a negativepotential with respect to a potential of the anode electrode; andreference electrode 19 that is disposed in the cathode chamber so as tobe in contact with the first electrolyte solution.

The electrolysis device according to the second exemplary embodiment issuitable because an electrolysis reaction can be controlled bycontrolling the potential of the cathode electrode, and an influence dueto temporal change on the anode electrode side can be eliminated.

As shown in FIGS. 1 and 2, the electrolysis device may have a tube inthe cathode chamber. A gaseous electrolysis reactant is supplied to thefirst electrolyte solution through the tube. Examples of the gaseouselectrolyte reactant include oxygen and carbon dioxide. Also as to aliquid/solid electrolysis reactant such as water, an inert gas can besupplied through a sub tube so as to suppress a side reaction. An end ofthe tube is immersed in the first electrolyte solution. The electrolysisdevice may also include a voltage measuring device and a currentmeasuring device for monitoring a state of a reduction reaction of anelectrolysis reactant.

The cathode electrode includes a region of a carbon fiber having astructure in which the carbon fiber contains a part of and/or a whole ofa catalyst particle (i.e., catalyst-containing structure). The catalystparticle is formed of, for example, a metal, oxide, carbide, nitride,boride, silicide, fluoride, or sulfide. Specific examples of thecatalyst particle include platinum, gold, silver, copper and a compoundthereof. The catalyst particle is not limited to employment of thesubstances exemplified above, but the constitution of the catalystparticle is not limited as long as an electrolysis product material canbe obtained by catalysis via the catalyst particle. A particle size ofthe catalyst particle can be freely set in a range not exceeding a fiberdiameter of the carbon fiber. In view of a specific surface area of thecatalyst, the catalyst particle size is preferably not more than 1 μm,more preferably not more than 100 nm. The fiber diameter of the carbonfiber can also be freely set in a range not being smaller than theparticle size of the catalyst particle. In view of the specific surfacearea described above, the fiber diameter of the carbon fiber ispreferably not more than 10 μm, more preferably not more than 1 μm.

Similarly, a concentration of the catalyst particle in the carbon fibercan also be freely set. A catalytic activity improves as theconcentration of the catalyst particle in the carbon fiber increasesbecause a surface area of particles exposed on a surface of the carbonfiber increases. However, when the concentration is too strong, strengthof the carbon fiber decreases, causing deterioration in catalyticactivity. The concentration of the catalyst particle in the carbon fiberranges preferably from 5 wt % to 30 wt %, more preferably from 5 wt % to20 wt %. Such a concentration of the catalyst particle can be, as adirect manner, obtained by heating in oxygen the carbon fiber having thecatalyst-containing structure to lose the carbon fiber and comparingweights before and after the heating (thermal analysis measurement).

Hereinafter, one example for producing a carbon fiber that is includedin a cathode electrode and includes a region having acatalyst-containing structure. However, the present disclosure is notlimited by the following production example at all.

The structure can be obtained by carbonizing a fiber formed from aprecursor of a carbon fiber, in which catalyst particles are dispersed.Examples of the precursor of a carbon fiber include polypropylene,polyethylene, polystyrene, polyethylene oxide, polyethyleneterephthalate, polybutylene terephthalate, polyethylene naphthalate,poly-m-phenylene terephthalate, poly-p-phenylene isophthalate,polyvinylidene fluoride, a polyvinylidene fluoride-hexafluoropropylenecopolymer, polyvinyl chloride, a polyvinylidene chloride-acrylatecopolymer, polyacrylonitrile, a polyacrylonitrile-methacrylatecopolymer, polycarbonate, polyacrylate, polyestercarbonate, a polyamide,an aramid, a polyimide, polycaprolactone, polyamic acid, polylacticacid, polyglycolic acid, collagen, polyhydroxy butyric acid, polyvinylacetate, a polypeptide, and a high polymer compound such as a copolymerthereof. One selected from the above examples may be used or a pluralityof kinds may be mixed. The carbon fiber precursor is not limited toemployment of the substances exemplified above.

The catalyst particles or the carbon fiber precursor may be dispersed ina solvent. Examples of the solvent in which the carbon fiber precursoris dispersed include methanol, ethanol, 1-propanol, 2-propanol,hexafluoroisopropanol, tetraethylene glycol, triethylene glycol,dibenzyl alcohol, 1,3-dioxolane, 1,4-dioxane, methyl ethyl ketone,methyl isobutyl ketone, methyl-n-hexyl ketone, methyl-n-propyl ketone,diisopropyl ketone, diisobutyl ketone, acetone, hexafluoroacetone,phenol, formic acid, methyl formate, ethyl formate, propyl formate,methyl benzoate, ethyl benzoate, propyl benzoate, methyl acetate, ethylacetate, propyl acetate, dimethyl phthalate, diethyl phthalate, dipropylphthalate, methyl chloride, ethyl chloride, methylene chloride,chloroform, o-chlorotoluene, p-chlorotoluene, carbon tetrachloride,1,1-dichloroethane, 1,2-dichloroethane, trichloroethane,dichloropropane, methyl bromide, ethyl bromide, propyl bromide, aceticacid, benzene, toluene, hexane, cyclohexane, cyclohexanone,cyclopentane, o-xylene, p-xylene, m-xylene, acetonitrile,tetrahydrofuran, N-methylpyrrolidone, N,N-dimethylformamide,N,N-dimethylacetamide, dimethyl sulfoxide, pyridine, and water. Oneselected from the above examples may be used or a plurality of kinds maybe mixed. The solvent is not limited to employment of the substancesexemplified above.

The catalyst particles in a powder state, or in a slurry state in whichthe catalyst particles are dispersed in the above solvent, are chargedinto the carbon fiber precursor and dispersed in the mixture. Theconcentration of the catalyst particles in the carbon fiber can becontrolled by adjusting a charging amount of the catalyst particles tobe dispersed in the carbon fiber precursor into the carbon fiberprecursor. Examples of the method for forming a fiber from the precursorinclude an electrospinning method and a wet spinning method.Carbonization is carried out in the atmosphere of an inert gas.Preferable examples of the inert gas include nitrogen and argon. Eachcarbon fiber precursor is preferably subjected to a pretreatment in anatmosphere and temperature conditions unique to the carbon fiberprecursor before the carbonization. As to a carbonization temperature,it is suitable to carry out the carbonization at a high temperature in arange not exceeding a melting point of the catalyst particles to improvemechanical strength of the carbon fiber and reduce resistance. In thecase of copper, for example, the carbonization is preferably carried outat 600° C. to 1000° C., more preferably 800° C. to 1000° C.

The cathode electrode may be formed of only the carbon fiber or may havea laminate structure of a base material for holding the carbon fiber anda conductive layer for increasing conductivity of the electrode. Thelaminate structure includes, for example, a carbon fiber/conductivelayer/base material structure, in which the conductive layer and thebase material should not be in contact with the electrolyte solution, orthe conductive layer and the base material should be formed of amaterial that is inert as a catalyst because the carbon fiber isstructurally impregnated with the electrolyte solution. Examples of theconductive layer that satisfies the conditions include a transparentelectrode membrane of ITO or the like, carbon, and various metals.Examples of the base material include glass, an epoxy resin, and acarbon substrate represented by Glassy Carbon (registered trademark).The carbon substrate is preferably used for achieving both theconductivity and catalytic inactivity. The carbon fiber preferablyadheres to the conductive layer for increasing electric characteristicsof the cathode electrode. Preferable examples of the adhesion methodinclude a method of pressure-adhesion of the carbon fiber with theconductive layer and a method of using a binder such as Nafion(registered trademark) available from E. I. du Pont de Nemours andCompany. The configuration of the cathode electrode is not particularlylimited as long as of the cathode electrode has a form that provides aneffect of reducing carbon dioxide.

The cathode electrode is in contact with the first electrolyte solution.More accurately, the cathode electrode includes a region of the carbonfiber having the catalyst-containing structure, and the region is incontact with the first electrolyte solution. It is sufficient that ifonly a part of the cathode electrode is immersed in the firstelectrolyte solution as long as the region is in contact with the firstelectrolyte solution.

The anode electrode includes a conductive substance. Examples of theconductive substance include carbon, platinum, gold, silver, copper,titanium, iridium oxide and an alloy thereof. A material of such aconductive substance is not particularly limited as long as theconductive substance is not decomposed by its own oxidation reaction.

An oxidation reaction of water on the anode electrode and a reductionreaction of carbon dioxide on the cathode electrode are differentindependent reaction systems, and the reaction that occurs on thecathode electrode side is never affected by a material of the anodeelectrode.

The anode electrode is in contact with the second electrolyte solution.More accurately, the conductive substance included in the anodeelectrode is in contact with the second electrolyte solution. It issufficient that if only a part of the anode electrode is immersed in thesecond electrolyte solution as long as the conductive substance is incontact with the second electrolyte solution.

The first electrolyte solution is stored in the cathode chamber. Thefirst electrolyte solution is an electrolyte solution having aprescribed concentration, and examples of the first electrolyte solutioninclude an aqueous potassium chloride solution, an aqueous sodiumchloride solution, and an aqueous potassium hydrogen carbonate solution.The second electrolyte solution is stored in the anode chamber. Thesecond electrolyte solution is an electrolyte solution having aprescribed concentration, and examples of the second electrolytesolution include an aqueous sodium hydroxide solution and an aqueouspotassium hydrogen carbonate solution.

The solid electrolyte membrane separates a chamber into the cathodechamber for storing the first electrolyte solution and the anode chamberfor storing the second electrolyte solution, and is necessary forpreventing components of the electrolyte solutions from mixing with eachother. The solid electrolyte membrane allows protons to passtherethrough so that the first electrolyte solution on the cathodeelectrode side is electrically connected to the second electrolytesolution on the anode electrode side. The solid electrolyte membrane is,for example a Nafion (registered trademark) membrane available from E.I. du Pont de Nemours and Company.

The reference electrode is for measuring the potential of the cathodeelectrode and is connected to the cathode electrode via a voltagemeasuring device. As the reference electrode, a silver/silver chlorideelectrode is used, for example.

The exemplary embodiments described above are a two-solution system inwhich the solid electrolyte membrane is used to separate the cathodechamber for storing the first electrolyte solution from the anodechamber for storing the second electrolyte solution. In the exemplaryembodiments, when an aqueous sodium chloride solution is used for boththe first electrolyte solution and the second electrolyte solution, forexample, an electrolysis product material is generated by anelectrolysis reaction on the cathode electrode side, while such anelectrode should be selected that does not generate a harmful chlorinegas on the anode electrode side. In a case of a one-solution system thatincludes no solid electrolyte membrane, a reverse reaction on thecathode electrode side occurs causing an electrolysis product materialgenerated to oxidize and return to an electrolysis reactant, thereforesuch an additional device that immediately removes an electrolysisproduct material from the reaction system is necessary by, for example,externally constructing a circulating system of the solution.

In the instant specification, the phrase “structure in which a carbonfiber contains a part of and/or a whole of a catalyst particle (i.e.,catalyst-containing structure)” means (I) the cathode electrode includesa carbon fiber, (II) the carbon fiber includes a first catalyst particleand a second catalyst particle, (III) an inside of the carbon fiber isfilled with carbon, (IV) at least a part of the first catalyst particleis located on a surface of the carbon fiber, and (V) the second catalystparticle is located in the inside of the carbon fiber so as to besurrounded by the carbon with which the inside of the carbon fiber isfilled.

More specifically, the cathode electrode includes carbon fiber 20 shownin FIG. 9A. Carbon fiber 20 includes first catalyst particle 21 andsecond catalyst particle 22. The inside of carbon fiber 20 is filledwith carbon. Needless to say, the surface of carbon fiber 20 is alsoformed of carbon.

At least a part of first catalyst particle 21 is located on the surfaceof carbon fiber 20. Other part of first catalyst particle 21 may beembedded in carbon fiber 20. In other words, a part of a region of firstcatalyst particle 21 is in contact with carbon fiber 20; however, theother part of the region of first catalyst particle 21 is exposed so asnot to be in contact with carbon fiber 20. Carbon fiber 20 may include aplurality of first catalyst particles 21.

On the other hand, second catalyst particle 22 is located in the insideof carbon fiber 20. In other words, second catalyst particle 22 issurrounded by the carbon with which the inside of carbon fiber 20 isfilled. FIG. 9B shows a cross-sectional view taken along the line 9B-9Bincluded in FIG. 9A. As shown in FIG. 9B, the whole surface of secondcatalyst particle 22 is in contact with the carbon with which the insideof carbon fiber 20 is filled. Carbon fiber 20 may include a plurality ofsecond catalyst particles 22.

Such carbon fiber 20 improves generation efficiency of the productmaterial. Carbon fiber 20 may be formed by an electrospinning method.

Method for Generating Electrolysis Product Material

A method for generating an electrolysis product material with theabove-mentioned electrolysis device is described.

The electrolysis device may be placed at room temperature andatmospheric pressure, and a high-pressure cell may also be used foraccelerating a reaction.

The external power source applies a voltage to the cathode electrode sothat the cathode electrode has a negative potential with respect to apotential of the anode electrode. A value of the voltage applied by theexternal power source has a threshold necessary for obtaining a reactionfor generating an electrolysis product material. This threshold variesdepending on, for example, a distance between the cathode electrode andthe anode electrode, a type of a material that constitutes the cathodeelectrode or the anode electrode, and a concentration of the firstelectrolyte solution.

A part of the voltage applied to the cathode electrode with respect tothe anode electrode is consumed for an oxidation reaction of water onthe anode electrode. By using configurations as shown in FIGS. 1 and 2,the voltage actually applied to the cathode electrode can be acquiredmore accurately. The potential of the cathode electrode with respect toa potential of the reference electrode varies depending on a type of amaterial that constitutes the reference electrode, and, for example, thepotential of the cathode electrode is preferably not more than −1.2 Vfor a reduction reaction of carbon dioxide, not more than −0.8 V for ahydrogen generation reaction, and not more than 0.6 V for an oxygenreduction reaction with respect to a silver/silver chloride electrode.

As described above, application of an appropriate voltage to the cathodeelectrode allows reduction of an electrolysis reactant contained in thefirst electrolyte solution on the cathode electrode. As a result, anelectrolysis product material is generated on the surface of the cathodeelectrode.

It is suitable to use the solid electrolyte membrane to separate theelectrolyte solution on the cathode side from the electrolyte solutionon the anode side.

A reaction current flows through the cathode electrode in response to areduction reaction of an electrolysis reactant on the surface of thecathode electrode and an oxidation reaction of water on the surface ofthe anode electrode in the electrolysis device. As shown in FIGS. 1 and2, incorporation of a current measuring device allows monitoring anamount of the reaction current.

EXAMPLES

The present disclosure is described in more detail with reference toExamples below.

Example 1 Production of Cathode Electrode

A cathode electrode for an electrolysis device, according to the presentdisclosure was produced. The cathode electrode includes a carbon fiberhaving a catalyst-containing structure.

First, a solution of polyamic acid in N-methylpyrrolidone (manufacturedby Ube Industries, Ltd., U-Vanish-A) was used as a precursor solution ofa carbon fiber. CuO particles (mean particle size: 20 nm) were used ascatalyst particles. As the catalyst particles, used were slurry catalystparticles (manufactured by CIK NanoTek Corporation, CUAP15 Wt %-G180)obtained by dispersing the catalyst particles in an organic solventmainly consisting of ethanol. The content of CuO in the slurry was 15%by weight ratio. The precursor solution and the catalyst particlesolution were mixed at a weight ratio of 9:1 to give a precursorsolution of a carbon fiber, in which the catalyst particles weredispersed.

The precursor solution of a carbon fiber, in which the catalystparticles were dispersed, was subjected to spinning by anelectrospinning method to form a fiber of the precursor. The spun fiberwas placed on a Glassy Carbon (diameter 25 mm×thickness 0.5 mm)substrate, and was subjected to a pretreatment in an argon (Ar)atmosphere according to a temperature profile shown in FIG. 3. Then, thetemperature was further raised to 800° C. in the Ar atmosphere and washeld for 30 minutes to carry out carbonization of the fiber.

As a result of observing a surface of the carbon fiber with a scanningelectron microscope, several tens of nm particles were confirmed to bepresent in the carbon fiber having a diameter ranges from severalhundred nm to several μm (FIG. 4A). As a result of observation with atransmission electron microscope, it was made apparent that the carbonfiber contained a part of and/or a whole of each particle (FIG. 4B).This particle was confirmed to be Cu by elemental analysis. It isconsidered that CuO was reduced in the Ar atmosphere to be Cu. As aresult of thermal analysis measurement, a concentration of the Cuparticles in the carbon fiber was found out to be about 12 wt %.

Then, the obtained carbon fiber/Glassy Carbon substrate with the carbonfiber having a catalyst-containing structure was bonded onto a glassplate with a metal sheet (aluminum) interposed between the substrate andthe plate. Subsequently, an exposed surface of the metal sheet wascovered with an epoxy resin for avoiding contact of the metal sheet withan electrolyte solution, to produce the cathode electrode according tothe present disclosure.

Assembly of Device

The electrolysis device shown in FIG. 2 was produced, the devicecomprising the cathode electrode described above. The configuration ofthe electrolysis device according to the present example is as follows.

Cathode electrode: carbon fiber/Glassy Carbon substrate (surface area: 5cm²) with the carbon fiber having a catalyst-containing structure

Anode electrode: platinum

Distance between electrodes: about 8 cm

Reference electrode: silver/silver chloride

Cathode side electrolyte solution: aqueous 0.5 mol/L potassium chloridesolution

Anode side electrolyte solution: aqueous 0.5 mol/L potassium hydrogencarbonate solution

Solid electrolyte membrane: Nafion membrane (manufactured by E. I. duPont de Nemours and Company, Nafion 424)

Supply of carbon dioxide to the cathode side electrolyte solution wascarried out by conducting a bubbling treatment of a carbon dioxide gas(flow rate of carbon dioxide: 200 mL/min) through a tube for 30 minutes.

A chamber was sealed after dissolution of carbon dioxide in the cathodeside electrolyte solution. Then, a voltage was applied to the cathodeelectrode with a potentiostat so that the cathode electrode had anegative potential with respect to a potential of the anode electrode. Avalue of the applied voltage was controlled by the potentiostat so thatthe cathode electrode had a potential of −1.8 V with respect to thereference electrode.

After the application of the voltage for a certain period of time, atype and an amount of a reaction product material generated in thecathode chamber were measured according to gas chromatography and liquidchromatography. As a result, hydrogen (H₂), carbon monoxide (CO), formicacid (HCOOH), methane (CH₄), ethylene (C₂H₄) aldehydes, and alcoholswere detected as reduction product materials of carbon dioxide. That is,a hydrocarbon represented by methane and ethylene was confirmed to begenerated by using for the cathode electrode the carbon fiber having thecarbon-containing structure. Further, as a result of observing a surfaceof the carbon fiber with a scanning electron microscope after theelectrolysis, catalyst particles were confirmed to be present in thecarbon fiber as shown in FIG. 5.

In Example 1, a generation amount of a hydrocarbon per electrolysis timeof 1000 seconds was 24.2 μmol. The electrolysis time is equal to a timeduring which the external power source applied a voltage to the cathodeelectrode. In addition, generation efficiency (Faradaic efficiency) of ahydrocarbon in Example 1 was not less than 50% and the hydrocarbon wasconfirmed to be selectively generated. Faradaic efficiency means a ratioof an electric charge used for generating a product material to a wholereaction electric charge, and is calculated by (Faradaic efficiency forgenerating product material)=(reaction electric charge used forgenerating product material)/(whole reaction electric charge)×100 [%].

Example 2

The same experiment as in Example 1 was carried out except forcontrolling the potentiostat to make the cathode electrode have apotential of −2.0 V with respect to the reference electrode.

As a result, a hydrocarbon was confirmed to be generated as a reductionproduct material of carbon dioxide. Further, it was also confirmed thatthe catalyst particles were present in the carbon fiber after theelectrolysis.

Example 3

The same experiment as in Example 1 was carried out except for using anaqueous 3 mol/L potassium chloride solution as the cathode sideelectrolyte solution.

As a result, a hydrocarbon was confirmed to be generated as a reductionproduct material of carbon dioxide. Further, it was also confirmed thatthe catalyst particles were present in the carbon fiber after theelectrolysis.

Example 4

The same experiment as in Example 1 was carried out except for using anaqueous 0.5 mol/L potassium hydrogen carbonate solution as the cathodeside electrolyte solution.

As a result, a hydrocarbon was confirmed to be generated as a reductionproduct material of carbon dioxide. Further, it was also confirmed thatthe catalyst particles were present in the carbon fiber after theelectrolysis.

Example 5

The same experiment as in Example 1 was carried out except for usingparticles having a mean particle size of 100 nm as the CuO particlesserving as the catalyst particles.

As a result, a hydrocarbon was confirmed to be generated as a reductionproduct material of carbon dioxide. Further, it was also confirmed thatthe catalyst particles were present in the carbon fiber after theelectrolysis.

Example 6

The same experiment as in Example 1 was carried out except for makingthe concentration of the catalyst particles in the carbon fiber 20% byweight ratio.

As a result, a hydrocarbon was confirmed to be generated as a reductionproduct material of carbon dioxide. Further, it was also confirmed thatthe catalyst particles were present in the carbon fiber after theelectrolysis.

Example 7

The same experiment as in Example 1 was carried out except for makingthe concentration of the catalyst particles in the carbon fiber 5% byweight ratio.

As a result, a hydrocarbon was confirmed to be generated as a reductionproduct material of carbon dioxide. Further, it was also confirmed thatthe catalyst particles were present in the carbon fiber after theelectrolysis.

Comparative Example 1

The same experiment as in Example 1 was carried out except for using, asthe cathode electrode, a carbon fiber/Glassy Carbon electrode with thecarbon fiber not including catalyst particles.

As a result, H₂, CO, and HCOOH were detected, while a hydrocarbon wasnot detected. That is, a hydrocarbon was not generated in ComparativeExample 1.

Comparative Example 2

The same experiment as in Example 1 was carried out except for using, asthe cathode electrode, a carbon fiber/Glassy Carbon electrode with thecarbon fiber including no catalyst particle and having Cu deposited onlyon a surface of the carbon fiber by a solution method. FIG. 6A is ascanning electron micrograph before the experiment in ComparativeExample 2.

As a result, the generation efficiency of a hydrocarbon decreased toabout a hundredth as compared to the case of Example 1. Further, as aresult of observing with a scanning electron microscope the surface ofthe carbon fiber after the experiment, the catalyst particles werehardly present as shown in FIG. 6B. It is considered that the catalystparticles on the surface of the carbon fiber were removed during theelectrolysis.

The generation efficiency of a hydrocarbon in Examples 1 to 6 andComparative Examples 1 and 2 described above is shown in FIG. 7. Asshown in FIG. 7, only in the case in which the carbon fiber/GlassyCarbon substrate with the carbon fiber having a catalyst-containingstructure was used as the cathode electrode, a hydrocarbon wasselectively generated. This shows that the catalyst particles containedin the carbon fiber selectively reduced carbon dioxide without beingremoved during the electrolysis.

Example 8

The same experiment as in Example 1 was carried out except for usinggold particles (mean particle size: 20 nm) as the catalyst particles.

As a result, carbon monoxide was confirmed to be generated as areduction product material of carbon dioxide. It was also confirmed thatthe catalyst particles were present in the carbon fiber after theelectrolysis.

A generation amount of carbon monoxide per unit time when the goldparticles were used is shown in FIG. 8A. A rate of generation of carbonmonoxide was about 67 times as high as the case in which gold particleswere not present (Comparative Example 1). This shows that the catalystparticles in the carbon fiber selectively reduced carbon dioxide andgenerated carbon monoxide.

Example 9

The same experiment as in Example 1 was carried out except for usinggallium oxide particles (mean particle size: 50 nm) as the catalystparticles.

As a result, formic acid was confirmed to be generated as a reductionproduct material of carbon dioxide. It was also confirmed that thecatalyst particles were present in the carbon fiber after theelectrolysis.

A generation amount of formic acid per unit time when the gallium oxideparticles were used is shown in FIG. 8B. A rate of generation of formicacid was about 11 times as high as the case in which gallium oxideparticles were not present (Comparative Example 1). This shows that thecatalyst particles in the carbon fiber selectively reduced carbondioxide and generated formic acid.

Example 10

The same experiment as in Example 1 was carried out except for usingplatinum particles (mean particle size: 20 nm) as the catalystparticles, conducting a bubbling treatment with Ar, and controlling thepotentiostat to make the cathode electrode have a potential of −0.8 Vwith respect to the reference electrode.

As a result, hydrogen was confirmed to be generated as a reductionproduct material of water. Further, it was also confirmed that thecatalyst particles were present in the carbon fiber after theelectrolysis.

Comparative Example 3

The same experiment as in Example 1 was carried out except for using, asthe cathode electrode, a carbon fiber/Glassy Carbon electrode with thecarbon fiber including no catalyst particle and having Pt deposited onlyon a surface of the carbon fiber by a solution method, and conducting abubbling treatment with Ar.

As a result, a generation amount of hydrogen per unit time decreased toabout a thirteenth as compared to the case of Example 10. Further, thecatalyst particles were hardly present on the surface of the carbonfiber after the experiment.

A generation amount of hydrogen per unit time when the platinumparticles were used is shown in FIG. 8C. A rate of generation ofhydrogen was about 50 times and 12 times as high as the case in whichplatinum particles were not present (Comparative Example 1) and the casein which platinum particles were present only on the surface of thecarbon fiber (Comparative Example 3), respectively. This shows that thecatalyst particles in the carbon fiber selectively reduced water withoutbeing removed during the electrolysis and generated hydrogen.

The present disclosure provides a novel cathode electrode, a noveldevice, and a novel method, for reducing an electrolysis reactant togenerate an electrolysis product material.

REFERENCE SIGNS LIST

-   100, 200 electrolysis device-   11 first electrolyte solution-   12 cathode chamber-   13 cathode electrode-   14 second electrolyte solution-   15 anode chamber-   16 solid electrolyte membrane-   17 anode electrode-   18 external power source-   19 reference electrode-   1 tube

What is claimed is:
 1. A method for reducing a reactantelectrochemically with an electrolysis device to generate a productmaterial, the method comprising: (a) preparing the electrolysis devicecomprising a cathode chamber, an anode chamber, a cathode electrode, ananode electrode, and a solid electrolyte membrane; wherein the cathodeelectrode includes a carbon fiber; the carbon fiber includes a firstcatalyst particle and a second catalyst particle; an inside of thecarbon fiber is filled with carbon; at least a part of the firstcatalyst particle is located on a surface of the carbon fiber; thesecond catalyst particle is located in the inside of the carbon fiber soas to be surrounded by the carbon with which the inside of the carbonfiber is filled; the anode electrode has a region formed of a metal ormetal compound; a first electrolyte solution is stored in the cathodechamber; a second electrolyte solution is stored in the anode chamber;the cathode electrode is in contact with the first electrolyte solution;the anode electrode is in contact with the second electrolyte solution;the first electrolyte solution contains the reactant; and the solidelectrolyte membrane separates the anode chamber from the cathodechamber; and (b) applying a voltage between the anode electrode and thecathode electrode to reduce the reactant on at least one of the firstcatalyst particle and the second catalyst particle.
 2. The methodaccording to claim 1, wherein the first catalyst particle and the secondcatalyst particle have a mean particle size of not more than 100nanometers.
 3. The method according to claim 1, wherein a ratio byweight of the first catalyst particle and the second catalyst particleto the carbon fiber is not less than 5% and not more than 20%.
 4. Themethod according to claim 1, wherein the electrolysis device furthercomprises a reference electrode of Ag/AgCl.
 5. The method according toclaim 1, wherein the region is formed of at least one selected from thegroup consisting of carbon, platinum, gold, silver, copper, titanium,iridium oxide, and an alloy thereof.
 6. An electrolysis device forreducing a reactant electrochemically to generate a product material,the electrolysis device comprising: a cathode chamber; an anode chamber;a cathode electrode; an anode electrode; a solid electrolyte membrane;and a power source, wherein the cathode electrode includes a carbonfiber; the carbon fiber includes a first catalyst particle and a secondcatalyst particle; an inside of the carbon fiber is filled with carbon;at least a part of the first catalyst particle is located on a surfaceof the carbon fiber; the second catalyst particle is located in theinside of the carbon fiber so as to be surrounded by the carbon withwhich the inside of the carbon fiber is filled; the anode electrode hasa region formed of a metal or metal compound; a first electrolytesolution is stored in the cathode chamber; a second electrolyte solutionis stored in the anode chamber; the cathode electrode is in contact withthe first electrolyte solution; the anode electrode is in contact withthe second electrolyte solution; the first electrolyte solution containsthe reactant; the solid electrolyte membrane separates the anode chamberfrom the cathode chamber; and the power source is capable of applying avoltage between the anode electrode and the cathode electrode.
 7. Theelectrolysis device according to claim 6, wherein the first catalystparticle and the second catalyst particle has a mean particle size ofnot more than 100 nanometers.
 8. The electrolysis device according toclaim 6, wherein a ratio by weight of the first catalyst particle andthe second catalyst particle to the carbon fiber is not less than 5% andnot more than 20%.
 9. The electrolysis device according to claim 6,wherein the electrolysis device further comprises a reference electrodeof Ag/AgCl.
 10. The electrolysis device according to claim 6, whereinthe anode electrode is formed of at least one selected from the groupconsisting of carbon, platinum, gold, silver, copper, titanium, iridiumoxide, and an alloy thereof.
 11. A cathode electrode used for reducing areactant electrochemically to generate a product material, the cathodeelectrode comprising: a carbon fiber, wherein the carbon fiber includesa first catalyst particle and a second catalyst particle; an inside ofthe carbon fiber is filled with carbon; at least a part of the firstcatalyst particle is located on a surface of the carbon fiber; and thesecond catalyst particle is located in the inside of the carbon fiber soas to be surrounded by the carbon with which the inside of the carbonfiber is filled.
 12. The cathode electrode according to claim 11,wherein the first catalyst particle and the second catalyst particlehave a mean particle size of not more than 100 nanometers.
 13. Thecathode electrode according to claim 11, wherein a ratio by weight ofthe first catalyst particle and the second catalyst particle to thecarbon fiber is not less than 5% and not more than 20%.